My Edelebrock carb seems to be real sensitive to temp and humidity. I had a simiar problem the other day when it was hot and humid. The engine didnt sound right and I was down 300 rpm but as soon as the sun went down she was roaring like a beast again. Put in some fresh plugs and check your jetting. Good luck I cant seem to find a happy medium.

Definitely start back over with the prop, then check the drive plugs for shavings in case there's a pending drive failure.
Are you sure the motor and drives are aligned good?
How about any water intrusion in the hull that may have added weight?
Throttle linkage adjusted proiperly to ensure you're getting real full throttle?
Are you sure the motor is making the same HP as before?

First, before spending anymore time. or $$$ Check compression. Then once you know that is ok. Look at prop.

Did they build the engine back to factory specs?

Did you check for an exhaust leak, steam coming out, bad seal causing frequency loss through exhaust and engine not breathing right.

If it isn't flat cam lobe, drive, failure etc. and they put engine back like stock, check your carb for vacuum leaks ( the little plugs around the tubes sticking off the carb) check plugs, check carb gaskets that seal between throttle linkage plate and manifold.

Leak down test will tell you if a spring has gotten weak and is not letting engine run best at WOT. It is possible for a valve to not be seating properly and allowing compresion leak up high. This will not show up very well on a regular cranking compresion test.

You could upgrade that engine though and get it moving. Oval port merlin heads with a 454-0 intake and 800 marine carb makes a night and day difference in that engine.

About 3k delivered from Jegs and an afternoon of work to put in. It will give you around 100hp with the stock cam.

Good luck with you issues.

Just though about the previous post about the driveline. Change your fluid and run it for a couple hours. take some fluid out and see if it smells burnt or rotten. THat will tell you maybe if a bearing is going out and slowing you down.

Thanks for all the input guys I will have to see about trying to test the "Leak Down" seeing i have done a compression test. I will see about someone helping me out with that one seeing i have no idea how to do that. :-) Thanks All for the input!
Jason

have you checked the firing order? it happened to me, two weeks before i found it.

try grounding individual spark plugs to see if one cylinder is not pulling its weight.

check fuel pressure at WOT, you could have an obstruction between the pickup and the pump.

Check the valve springs as well. Jas